Platinum electrode surface coating and method for manufacturing the same

ABSTRACT

An electrode surface coating and method for manufacturing the electrode surface coating comprising a conductive substrate; a surface coating of platinum having a rough configuration and an increase in the surface area of 5 times to 500 times of the corresponding surface area resulting from the basic geometric shape of the electrode. A method for electroplating an electrode surface with platinum coating having a rough surface, comprising electroplating the surface of a conductive substrate at a rate such that the metal particles form on the conductive substrate faster than necessary to form shiny platinum and slower than necessary to form platinum black.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This patent application is a divisional application of U.S. patentapplication Ser. No. 11/259,822, filed Oct. 26, 2005 the disclosure ofwhich is incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant No.:R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to platinum electrode surface coatingand electroplating processes for deposition of surface coating.

2. Description of Related Art

Platinum has often been used as a material for electrodes in corrosiveenvironments such as the human body due to its superior electricalcharacteristics, biocompatibility and stability. Platinum has manydesirable qualities for use as an electrode for electrical stimulationof body tissue. Since platinum has a smooth surface and its surface areais limited by the geometry of the electrode, it is not efficient fortransferring electrical charge. The platinum with a smooth surface ishereinafter called “shiny platinum”.

Electrodes for stimulating body tissue by electrical stimulation areknown in great variety. For the utility of an implantable stimulation orsensing electrode—especially one intended for long-term use in a tissuestimulator with a non-renewable energy source and that, therefore, mustrequire minimal energy—a high electrode capacitance and correspondinglylow electrical impedance is of great importance. Furthermore, withoutsufficiently low impedance, a large voltage may cause polarization ofboth the electrode and the tissue to which the electrode is attachedforming possibly harmful byproducts, degrading the electrode anddamaging the tissue.

Because the ability of an electrode to transfer current is proportionalto the surface area of the electrode and because small electrodes arenecessary to create a precise signal to stimulate a single nerve orsmall group of nerves, many in the art have attempted to improve theability of an electrode to transfer charge by increasing the surfacearea of the electrode without increasing the size of the electrode.

One approach to increase the surface area of a platinum electrodewithout increasing the electrode size and therefore to improve theability of the electrode to transfer charge is to electroplate platinumrapidly such that the platinum molecules do not have time to arrangeinto a smooth, shiny surface. The rapid electroplating forms a platinumsurface which is commonly known as “platinum black”. Platinum black hasa porous and rough surface which is less dense and less reflective thanshiny platinum. U.S. Pat. No. 4,240,878 to Carter describes a method ofplating platinum black on tantalum.

Platinum black is more porous and less dense than shiny platinum.Platinum black has weak structural and physical strength and istherefore not suitable for applications where the electrode is subjectto even minimal physical stresses. Platinum black also requiresadditives such as lead to promote rapid plating. Lead, however, is aneurotoxin and cannot be used in biological systems. Finally, due toplatinum black's weak structure, the plating thickness is quite limited.Thick layers of platinum black simply fall apart.

The US patent application No. 2003/0192784 “Platinum Electrode andMethod for Manufacturing the Same” to Dao Min Zhou filed Aug. 23, 2002,the disclosure of which is incorporated herein by reference, disclosesan electrode comprising an electrode body; and a surface coating ofplatinum having a fractal configuration.

The US patent application No. 2003/0233134 “Biocompatible Bonding Methodand Electronics Package Suitable for Implantation” to Robert J.Greenberg et al. filed Sep. 6, 2002, the disclosure of which isincorporated herein by reference, discloses a device comprising asubstrate at least a portion of which is electrically conductive; aflexible assembly; and at least one deposited rivet that bonds saidsubstrate and said flexible assembly together.

The US patent application No. 2004/0220652 “Adherent Metal Oxide CoatingForming a High Surface Area Electrode” to Dao Min Zhou et al. filed Nov.4, 2004, the disclosure of which is incorporated herein by reference,discloses an implantable electrode comprising: a roughened conductivesubstrate; and a rough surface coating on said substrate.

There is a further need for an alternative electroplating of a thinnersurface with platinum to obtain an increased surface area for a givengeometry and at the same time the coating is structurally strong enoughto be used in applications where the platinum surface coating is subjectto physical stresses.

SUMMARY OF THE INVENTION

The present invention is directed in part to an electrode surfacecoating having increased surface area for greater ability to transfercharge and also having sufficient physical and structural strength towithstand physical stress.

This and other aspects of the present invention which may become obviousto those skilled in the art through the following description of theinvention are achieved by an improved surface coating and method forpreparing the improved surface coating having a rough surface comprisingone or more of the following metals titanium, niobium, tantalum,ruthenium, rhodium, iridium, palladium, platinum, or gold, or an alloyof two or more metals, or a combination of two or more alloys or metallayers thereof having an increase in the surface area of 5 times to 500times of the corresponding surface area resulting from the basicgeometric shape.

The coating of the present invention has an increase in surface area ofat least 5 times compared to shiny platinum of the same geometry. Thecoating of the present invention has at the same time an improvedresistance to physical stress when compared to platinum black.

The process of electroplating the surface coating of the presentinvention comprises plating at a rate that is faster than the ratenecessary to produce shiny platinum and that is less than the ratenecessary to produce platinum black.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a platinum gray surface magnified 5000 times.

FIG. 2 shows a shiny platinum surface magnified 2000 times.

FIG. 3 shows a platinum black surface magnified 2000 times.

FIG. 4 shows SEM of an electroplated platinum rough surface at −0.525 Vmagnified 5000 times.

FIG. 5 shows SEM of an electroplated platinum rough surface at −0.5 Vmagnified 5000 times.

FIG. 6 shows color density (D) values and lightness (L*) values forseveral representative samples of platinum gray, platinum black andshiny platinum.

FIG. 7 shows a three-electrode electroplating cell with a magneticstirrer.

FIG. 8 shows a three-electrode electroplating cell in an ultrasonictank.

FIG. 9 shows a three-electrode electroplating cell with a gas dispersiontube.

FIG. 10 shows an electroplating system with constant voltage control orconstant current control.

FIG. 11 shows an electroplating system with pulsed current control.

FIG. 12 shows an electroplating system with pulsed voltage control.

FIG. 13 shows an electroplating system with scanned voltage control.

FIG. 14 shows an electrode platinum silicone array having 16 electrodes.

FIG. 15 shows the electrode capacitance for both plated and unplatedelectrodes of varying diameter.

FIG. 16 shows a representative linear voltage sweep of a representativeplatinum gray electrode.

FIG. 17 shows a representative linear voltage sweep of a representativerough platinum electrode.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed in part to an electrode surfacecoating having increased surface area for greater ability to transfercharge and also having sufficient physical and structural strength towithstand physical stress.

The electrode surface coating is achieved by an improved surface coatingand method for preparing the improved surface coating having a roughsurface comprising platinum having a rough configuration and an increasein the surface area of 5 times to 500 times of the corresponding surfacearea resulting from the basic geometric shape of the electrode.

The conductive substrate comprises one or more of the following metalstitanium, zirconium, niobium, tantalum, chromium, molybdenum, tungsten,manganese, rhenium, ruthenium, rhodium, iridium, nickel, palladium,platinum, silver, gold or carbon.

The conductive substrate comprises preferably palladium, platinum,iridium, gold, tantalum, titanium, titanium nitride or niobium.

The conductive substrate preferably comprises one or more of thefollowing metals platinum, platinum alloy, iridium, iridium oxide orrhodium.

The process of electroplating an electrode surface coating having arough surface comprises electroplating the surface of a conductivesubstrate at a rate such that the metal particles form on the conductivesubstrate faster than necessary to form shiny metal and slower thannecessary to form metal black.

FIGS. 1, 2, and 3 are images produced on a Scanning Electron Microscope(SEM). FIG. 1 at 5000× and FIGS. 2 and 3 at 2000× are magnificationstaken by a JEOL JSM5910 microscope (Tokyo, Japan). FIG. 1 shows anillustrative example of a platinum gray surface coating for an electrodehaving a fractal surface with a surface area increase of greater than 5times the surface area for a shiny platinum surface of the samegeometry, shown in FIG. 2, and an increase in strength over a platinumblack surface, shown in FIG. 3. Under the magnification level of 5000 itis observed that platinum gray is of a fractal configuration having acauliflower shape with particle sizes ranging from 0.5 μm to 15 μm. Eachbranch of such structure is further covered by smaller and smallerparticles of similar shape. The smallest particles on the surface layermay be in the nanometer range. This rough and porous fractal structureincreases the electrochemically active surface area of the platinumsurface when compared to an electrode with a smooth platinum surfacehaving the same geometric shape.

The surface is pure platinum because no impurities or other additivessuch as lead need to be introduced during the plating process to produceplatinum gray. This is especially advantages in the field of implantableelectrodes because lead is a neurotoxin and cannot be used in theprocess of preparing implantable electrodes. Alternatively, othermaterials such as iridium, rhodium, gold, tantalum, titanium or niobiumcould be introduced during the plating process if so desired but thesematerials are not necessary to the formation of platinum gray.

Platinum gray can also be distinguished from platinum black and shinyplatinum by measuring the color of the material on a spectrodensitometerusing the Commission on Illumination L*a*b* color scale. L* defineslightness, a* denotes the red/green value and b*, the yellow/blue value.The lightness value (called L* Value) can range from 0 to 100, wherewhite is 100 and black is 0—similar to grayscale. The a* value can rangefrom +60 for red and −60 for green, and the b* value can range from +60for yellow and −60 for blue. All samples measured have very small a* andb* values (they are colorless or in the so called neutral gray zone),which suggests that the lightness value can be used as grayscale forPlatinum coatings.

FIG. 4 shows an illustrative example of a platinum surface coating foran electrode having a rough surface with a surface area increase ofgreater than 5 times the surface area for a platinum surface of the samegeometry. Under the magnification of 5000 times it is observed that theplatinum is of a rough configuration having a regular shape withparticle sizes ranging from 0.1 μm to 2.0 μm and has an average size of0.4 μm to 0.6 μm, preferably about 0.5 μm. The thickness of the coatingis 0.1 μm to 5.0 μm, preferably 1.0 μm to 4.0 μm, more preferably 3.3 μmto 3.8 μm. Some rough features with a scale in the nanometer range werepresent on each particle. The plated platinum layer is not believed tobe porous. The bead shaped platinum particles with nanometer roughfeatures on the particles increase the electrode's electrochemicalactive surface. The electrochemical capacitance of the electrode arraywith the surface coating of rough platinum is about 1300 μF/cm² to 1500μF/cm², measured in a 10 mM phosphate buffered saline solution. Therelation of the platinum surface area to the thickness of the platinumsurface coating is of 4.0 F/cm³ to 5.0 F/cm³. The thin-film platinumdisks have an average capacitance of less than 20 μF/cm² before plating,measured at the same condition. The electrochemical active surface areaincrease is 70 to 80, preferably about 71 to 75 fold.

The SEM/EDX analysis confirms that plated surface contains pureplatinum. The surface is pure platinum because no impurities or otheradditives such as lead need to be introduced during the plating processto produce this platinum.

The electroplating process with platinum can be preferably performed inan agues solution containing sodium dihydrogen phosphate (NaH₂PO₄)and/or disodium hydrogen phosphate (Na₂HPO₄) and platinum tetra chloride(PtCl₄) at 20° C. to 40° C. Different concentrations of platinum can beused and the range of platinum salt concentrations can be from 1 to 30mM. Other Pt salts will also produce similar results.

FIG. 5 shows an illustrative example of a platinum surface coating foran electrode having a rough surface with a surface area increase ofgreater than 5 times the surface area for a platinum surface of the samegeometry. Under the magnification of 5000 times it is observed that theplatinum is of a rough configuration having a regular shape withparticle sizes ranging from 0.1 μm to 2.0 μm and has an average size of0.4 μm to 0.6 μm, preferably about 0.5 μm. The thickness of the coatingis 0.1 μm to 4.0 μm, preferably 2.0 μm to 3.0 μm, more preferably 2.3 μmto 2.8 μm. Some rough features with a scale in the nanometer range werepresent on each particle. The plated platinum layer is not believed tobe porous. The bead shaped platinum particles with nanometer roughfeatures on the particles increased electrode's electrochemical activesurface. The electrochemical capacitance of the electrode array with thesurface coating of rough platinum is about 1150 μF/cm² to 1680 μF/cm²,measured in a 10 mM phosphate buffered saline solution. The relation ofthe platinum surface area to the thickness of the platinum surfacecoating is of 5.0 F/cm³ to 6.0 F/cM³. The thin-film platinum disks havean average capacitance of less than 20 μF/cm² before plating, measuredat the same condition. The electrochemical active surface area increaseis 65 to 75, preferably about 68 to 72 fold.

The SEM/EDX analysis confirms that plated surface contains pureplatinum. The surface is pure platinum because no impurities or otheradditives such as lead need to be introduced during the plating processto produce this platinum.

The relation of the platinum surface area to the thickness of theplatinum surface coating is of 4.0 F/cm³ to 5.0 F/cm³ as referred to inFIG. 4 and of 5 5.0 F/cm³ to 6.0 F/cm³ as referred to in FIG. 5. FIGS. 4and 5 both depict a rough surface platinum coating according to thepresent invention. This value is calculated by dividing theelectrochemically active platinum surface area (μF/cm³) by the thicknessof the platinum coating (μm). In comparison to the rough surfaceplatinum coating the fractal platinum coating as referred to in FIG. 1has a relation of the platinum surface area to the thickness of theplatinum surface coating of 0.8 F/cm³ to 1.5 F/cm³. The rough platinumcoating of the present invention yields on the same electrochemicallyactive surface area a thinner coating with a higher capacitive volumecompared with platinum gray.

FIG. 6 shows the L*, a*, and b* values for representative samples ofplatinum gray, platinum black and shiny platinum as measured on a colorreflection spectrodensimeter, X-Rite 520. Platinum gray's L* valueranges from 25 to 90, while platinum black and shiny platinum both haveL* values less than 25. Measurements of color densities forrepresentative samples of platinum gray, platinum black and shinyplatinum are shown in FIG. 6 as well. Platinum gray's color densityvalues range from 0.4D to 1.3D; while platinum black and shiny platinumboth have color density values greater than 1.3D.

Platinum gray can also be distinguished from platinum black based on theadhesive and strength properties of the thin film coating of thematerials. Adhesion properties of thin film coatings of platinum grayand platinum black on 500 μm in diameter electrodes have been measuredon a Micro-Scratch Tester (CSEM Instruments, Switzerland). A controlledmicro-scratch is generated by drawing a spherical diamond tip of radius10 μm across the coating surface under a progressive load from 1 mN to100 mNs with a 400 μm scratch length. At a critical load the coatingwill start to fail. Using this test it is found that platinum gray canhave a critical load of over 60 mNs while platinum black has a criticalload of less than 35 mNs.

FIGS. 7, 8, 9 and 10 show a method to produce platinum gray according tothe present invention comprising connecting a platinum electrode 2, theanode, and a conductive substrate to be plated 4, the cathode, to apower source 6 with a means of controlling and monitoring 8 either thecurrent or voltage of the power source 6. The anode 2, cathode 4, areference electrode 10 for use as a reference in controlling the powersource 6 and an electroplating solution are placed in an electroplatingcell 12 having a means 14 for mixing or agitating the electroplatingsolution. Power is supplied to the electrodes with constant voltage,constant current, pulsed voltage, scanned voltage or pulsed current todrive the electroplating process. The power source 6 is modified suchthat the rate of deposition will cause the platinum to deposit asplatinum gray, the rate being greater than the deposition rate necessaryto form shiny platinum and less than the deposition rate necessary toform platinum black.

FIGS. 7, 8 and 9 show that the electroplating cell 12, is preferably a50 ml to 150 ml four neck glass flask or beaker, the common electrode 2,or anode, is preferably a large surface area platinum wire or platinumsheet, the reference electrode 10 is preferably a Ag/AgCl electrode(silver, silver chloride electrode), the conductive substrate to beplated 4, or cathode, can be any suitable material depending on theapplication and can be readily chosen by one skilled in the art.Preferable examples of the conductive substrate to be plated 4 includebut are not limited to platinum, iridium, rhodium, gold, tantalum,titanium or niobium.

The stirring mechanism is preferably a magnetic stirrer 14 as shown inFIG. 7, an ultrasonic tank 16 (such as the VWR Aquasonic 50D) as shownin FIG. 8, or gas dispersion 18 with Argon or Nitrogen gas as shown inFIG. 7. The plating solution is preferably 3 to 30 mM ammoniumhexachloroplatinate in disodium hydrogen phosphate, but may be derivedfrom any chloroplatinic acid or bromoplatinic acid or otherelectroplating solution. The preferable plating temperature isapproximately 24 ° C. to 26 ° C.

Electroplating systems with pulsed current and pulsed voltage controlare shown in FIGS. 9 and 10 respectively. While constant voltage,constant current, pulsed voltage or pulsed current can be used tocontrol the electroplating process, constant voltage control of theplating process has been found to be most preferable. The mostpreferable voltage range to produce platinum gray has been found to be−0.45 Volts to −0.85 Volts. Applying voltage in this range with theabove solution yields a plating rate in the range of about 1 μm perminute to 0.05 μm per minute, the preferred range for the plating rateof platinum gray. Constant voltage control also allows an array ofelectrodes in parallel to be plated simultaneously achieving a fairlyuniform surface layer thickness for each electrode.

The optimal potential ranges for platinum gray plating are solution andcondition dependent. Linear voltage sweep can be used to determine theoptimal potential ranges for a specific plating system. A representativelinear voltage sweep is shown in FIG. 16 for the fractal platinum grayas surface and in FIG. 17 for non fractal rough platinum surfaceaccording to FIGS. 4 and 5. During linear voltage sweep, the voltage ofan electrode is scanned cathodically until hydrogen gas evolution occurswhich reveals plating rate control steps of electron transfer 20 anddiffusion 22. For a given plating system, it is preferable to adjust theelectrode potential such that the platinum reduction reaction has alimiting current under diffusion control or mixed control 24 betweendiffusion and electron transfer but that does not result in hydrogenevolution 26.

It is very difficult to plate shiny platinum in layers greater thanapproximately several microns because the internal stress of the denseplatinum layer which will cause the plated layer to peel off and theunderlying layers cannot support the above material. The additionalthickness of the plate's surface layer allows the electrode to have amuch longer usable life.

The following example is illustrative of electroplating platinum on aconductive substrate to form a platinum surface coating.

Electrodes with a surface layer of platinum are prepared in thefollowing manner using constant voltage plating. An electrode platinumsilicone array having 16 electrodes where the diameter of the platinumdiscs on the array range from 510 μm to 530 μm, as shown in FIG. 14, isfirst cleaned electrochemically in sulfuric acid and the startingelectrode impedance is measured in phosphate buffered saline solution.Referring to FIG. 7, the electrodes are arranged in the electroplatingcell such that the plating electrode 2 is in parallel with the commonelectrode 4. The reference electrode 10 is positioned next to theelectrode 4. The plating solution is added to the electroplating cell 12and the stirring mechanism 14 is activated.

A constant voltage is applied on the plating electrode 2 as compared tothe reference electrode 10 using an EG&G PAR M273 potentiostat 6. Theresponse current of the plating electrode 2 is recorded by a recordingmeans 8. The response current is measured by the M273 potentiostat 6.After a specified time, preferably 1-90 minutes, and most preferably 30minutes, the voltage is terminated and the electrode 4 is thoroughlyrinsed in deionized water.

The electrochemical impedance of the electrode array with the surfacecoating of platinum is measured in a saline solution. The charge/chargedensity and average plating current/current density are calculated byintegrating the area under the plating current vs. time curve. ScanningElectron Microscope (SEM)/Energy Dispersed Analysis by X-ray (EDAX™)analysis can be performed on selected electrodes. SEM Micrographs of theplated surface can be taken showing its fractal or rough surface. EnergyDispersed Analysis demonstrates that the sample is pure platinum ratherthan platinum oxide or some other materials.

The voltage range is most determinative of the formation of the fractalsurface of platinum gray as shown in FIG. 1. For this system it isobserved that the preferred voltage drop across the electrodes toproduce platinum gray is approximately −0.55 to −0.65 Volts vs. Ag/AgClreference electrode 10. The preferred platinum concentration for theplating solution is observed to be approximately 8 to 18 mM ammoniumhexachloroplatinate in 0.4 M (Mole) disodium hydrogen phosphate.

The voltage range is determinative of the formation of the rough surfaceof platinum as shown in FIGS. 4 and 5. For this system it is observedthat the preferred voltage drop across the electrodes to produce roughplatinum is approximately −0.45 to −0.55 Volts vs. Ag/AgCl referenceelectrode 10. The preferred platinum concentration for the platingsolution is observed to be approximately 3 mM to 30 mM PtCl₄ in sodiumdihydrogen phosphate.

FIG. 14 provides a perspective view of a retinal electrode array for usewith the present invention, generally designated 32, comprisingoval-shaped electrode array body 34, a plurality of electrodes 36 madeof a conductive material, such as platinum or one of its alloys, butthat can be made of any conductive biocompatible material such asiridium, iridium oxide or titanium nitride, and a single referenceelectrode 38 made of the same material as electrode 36, wherein theelectrodes are individually attached to separate conductors 40 made of aconductive material, such as platinum or one of its alloys, but whichcould be made of any biocompatible conductive material, that isenveloped within an insulating sheath 42, that is preferably silicone,that carries an electrical signal to each of the electrodes 36.

A strain relief internal tab 44, defined by a strain relief slot 46 thatpasses through the array body 34, contains a mounting aperture 48 forfixation of the electrode array body 34 to the retina of the eye orother neural interface by use of a surgical tack. A reinforcing ring 50is colored and opaque to facilitate locating the mounting aperture 48during surgery. A grasping handle 52 is located on the surface ofelectrode array body 34 to enable its placement by a surgeon usingforceps or by placing a surgical tool into the hole formed by graspinghandle 52. Grasping handle 52 avoids damage to the electrode body thatmight be caused by the surgeon grasping the electrode body directly. Theelectrode array 32 is described in greater detail in US PatentApplication No. 2002/0111658 A1 filed Feb. 13, 2001 and entitledImplantable Retinal Electrode Array Configuration for Minimal RetinalDamage and Method of Reducing Retinal Stress, which is incorporatedherein by reference.

FIG. 15 shows the increase in electrode capacitance of severalelectrodes of varying diameter for a polyimide array plated according tothe above example at −0.6 V vs. Ag/AgCl Reference electrode for 30minutes compared with unplated electrodes of the same diameters. Becausethe electrode capacitance is proportional to its surface area, thesurface area increase, calculated from electrode capacitance, is 60 to100 times that of shiny platinum for this array. It should be noted thatshiny platinum exhibits some roughness and has a surface area increaseup to 3 times that of the basic geometric shape. While it is simple tomeasure a surface area change between two sample using capacitance, itis difficult to compare a sample with the basic geometric shape.

As plating conditions, including but not limited to the platingsolution, surface area of the electrodes, pH, platinum concentration andthe presence of additives, are changed the optimal controlling voltageand/or other controlling parameters will also change according basicelectroplating principles. Platinum gray will still be formed so long asthe rate of deposition of the platinum particles is slower than that forthe formation of platinum black and faster than that for the formationof shiny platinum.

The present invention provides a platinum coating with very regularparticle shape and regular average size. The coating is thinner thanknown platinum coatings and has a rough surface which is mainly notporous with a large surface area. The coating of the present inventionprovides a good adherence between the substrate and the platinumcoating. The platinum coated electrode is biocompatible and thereforeimplantable and provides less tissue reaction.

The present invention will be further explained in detail by thefollowing examples.

EXAMPLE 1 Electroplating Platinum on a Conductive Substrate PlatinumPlating Solution Preparation

0.3 g sodium dihydrogen phosphate (NaH₂PO₄) and 6.03 g disodium hydrogenphosphate (Na₂HPO₄) [Fluka] were dissolved in 100 ml deionized water,and stirred by magnetic stirring for 30 minutes. The concentrations forNaH₂PO₄ and Na₂HPO₄ were 25 mM and 425 mM. Then 0.5 g of Platinumchloride (PtCl₄) [Alfa Aesar] was added to the phosphate solution toform the platinum salt concentrations of 15 mM. The solution was thenstirred for 30 minutes. Different concentrations of (PtCl₄) were used inthe experiments and the range of Pt salt concentrations was from 3 to 30mM. The pH of the solution was measured at 7.9. The color of thesolution was amber. The solution was deaerated before the platingprocess by bubbling nitrogen through the solution.

Preparation of the Substrate

A thin-film platinum polyimide array was used for platinum plating. Thearray included 16 electrodes with 200 um thin-film Pt disk as exposedelectrode surface. All the electrodes in the array were shorted tocommon contact points for the plating. The Pt disk electrodes were firstelectrochemically cleaned by bubbling the surface with oxygen at +2.8Vvs Ag/AgCl in 0.5 M H₂SO₄ for 10 sec. Then the surface was cleaned bybubbling with hydrogen at −1.2 V vs Ag/AgCl in 0.5 M H₂SO₄ for 15 sec toremove surface contaminations and polymer residues.

Electroplating Cell

A classical Pyrex glass three-electrode reaction cell was used for theelectroplating. The reference electrode compartment was separated fromthe reaction compartment by a Vicor porous frit, in order to avoid themigration of concentrated KCl and AgCl from the inner filling solutionof the reference electrode to the plating bath. The counter electrodewas a platinized-platinum sheet of a real surface area equal to 1.8 cm².

A digital magnetic stirrer (Dataplate PMC720) was used to agitate thesolution during plating. The solution temperatures were from 15° C. to80° C. and were controlled by a VWR circulating water bath with adigital temperature controller (VWR 1147P).

The potential was controlled by using an EG&G PARC model 273potentiostat-galvanostat and the response current, current density andcharge were recorded by EG&G PARC M270 software. The charge/chargedensity and average plating current/current density were calculated byintegrating the area under the plating current vs. time curve. Theplating time was from 1 minute to 60 minutes.

Platinum Plating

A platinum polyimide electrode array having 16 electrodes (FIG. 14)having a diameter of 200 μm platinum disc on the array was cleanedelectrochemically in 0.5 M H₂SO₄. The electrode array was placed in anelectroplating cell containing a plating solution having a concentrationof 15 mM platinum chloride in 0.025 M sodium dihydrogen phosphate and0.425 M disodium hydrogen phosphate. The plating bath temperature was at22° C. A constant voltage of −0.525 V vs Ag/AgCl reference electrode wasapplied on the electrode and terminated after 10 minutes. The electrodearray was thoroughly rinsed in deionized water. The charge/chargedensity and average plating current/current density were calculated byintegrating the area under the plating current vs. time curve. Thecurrent density was near linearly increased from initial 11.1 A/cm² tofinal 15.2 A/cm². The electrochemical capacitance of the electrode arraywith the surface coating of rough platinum was 1462 μF/cm², measured ina 10 mM phosphate buffered saline solution. The thin-film Pt disks onlyhave an average capacitance of less than 20 μF/cm² before platingmeasured at the same condition. The optimal voltage drop across theelectrodes for producing rough platinum was from −0.4 to −0.7 Volts vs.Ag/AgCl reference electrode. The plated platinum surface coatingthickness is about 3.5 μm. The electrochemical active surface areaincrease is about 73 fold. The relation of surface area to the thicknessof the platinum surface coating is 4.18 F/cm³ [surface coating of roughplatinum 1462 μF/cm² per thickness of the platinum coating of 3.5 μm.]The platinum surface coating adhesive strength was 55 mN.

Example 1 yields a platinum surface coating having a rough surface asshown in FIG. 4. The platinum coating contains particles with veryregular particle shape and regular average size. The coating is thinnerthan known platinum coatings and has a rough surface which is mainly notporous with a large surface area. The coating provides a good adherencebetween the substrate and the platinum coating. The platinum coatedelectrode is biocompatible and therefore implantable and provides lesstissue reaction.

EXAMPLE 2 Electroplating Platinum on a Conductive Substrate PlatinumPlating Solution Preparation

0.3 g sodium dihydrogen phosphate (NaH₂PO₄) and 6.03 g disodium hydrogenphosphate (Na₂HPO₄) [Fluka] were dissolved in 100 ml deionized water,and stirred by magnetic stirring for 30 minutes. The concentrations forNaH₂PO₄ and Na₂HPO₄ were 25 mM and 425 mM. Then 0.5 g of Platinumchloride (PtCl₄) [Alfa Aesar] was added to the phosphate solution toform the platinum salt concentrations of 15 mM. The solution was thenstirred for 30 minutes and filtered to black solids. Differentconcentrations of (PtCl₄) were used in the experiments and the range ofPt salt concentrations was from 3 to 30 mM. The pH of the solution wasmeasured at 7.9. The color of the solution was amber. The solution wasdeaerated before the plating process by bubbling nitrogen through thesolution.

Preparation of the Substrate

A thin-film platinum polyimide array was used for platinum plating. Thearray included 16 electrodes with 200 μm thin-film Pt disk as exposedelectrode surface. All the electrodes in the array were shorted tocommon contact points for the plating. The Pt disk electrodes were firstelectrochemically cleaned by bubbling the surface with oxygen at +2.8Vvs Ag/AgCl in 0.5 M H₂SO₄ for 10 sec. Then the surface was cleaned bybubbling with hydrogen at −1.2 V vs Ag/AgCl in 0.5 M H₂SO₄ for 15 sec toremove surface contaminations and polymer residues.

Electroplating Cell

A classical Pyrex glass three-electrode reaction cell was used for theelectroplating. The reference electrode compartment was separated fromthe reaction compartment by a Vicor porous frit, in order to avoid themigration of concentrated KCl and AgCl into the inner filling solutionof the reference electrode. The counter electrode was aplatinized-platinum sheet of a real surface area equal to 1.8 cm².

A digital magnetic stirrer (Dataplate PMC720) was used to agitate thesolution during plating. The solution temperatures were from 15° C. to80° C. and were controlled by a VWR circulating water bath with adigital temperature controller (VWR 1147P).

The potential was controlled by using an EG&G PARC model 273potentiostat-galvanostat and the response current, current density andcharge were recorded by EG&G PARC M270 software. The charge/chargedensity and average plating current/current density were calculated byintegrating the area under the plating current vs. time curve. Theplating time was from 1 minute to 60 minutes.

Platinum Plating

A platinum polyimide electrode array having 16 electrodes (FIG. 14)having a diameter of 200 μm platinum disc on the array was cleanedelectrochemically in 0.5 M H₂SO₄. The electrode array was placed in anelectroplating cell containing a plating solution having a concentrationof 15 mM platinum chloride in 0.025 M sodium dihydrogen phosphate and0.425 M disodium hydrogen phosphate. The plating bath temperature was at22° C. A constant voltage of −0.5 V vs Ag/AgCl reference electrode wasapplied on the electrode and terminated after 10 minutes. The electrodearray was thoroughly rinsed in deionized water. The charge/chargedensity and average plating current/current density were calculated byintegrating the area under the plating current vs. time curve. Thecurrent density was near linearly increased from initial 10.8 A/cm² tofinal 14.6 A/cm². The electrochemical capacitance of the electrode arraywith the surface coating of rough platinum was 1417 μF/cm², measured ina 10 mM phosphate buffered saline solution. The thin-film Pt disks onlyhave an average capacitance of less than 20 μF/cm² before platingmeasured at the same condition. The optimal voltage drop across theelectrodes for producing rough platinum was from −0.4 to −0.7 Volts vs.Ag/AgCl reference electrode. The plated platinum surface coatingthickness is about 2.5 μm. The electrochemical active surface areaincrease is about 70 fold. The relation of surface area to the thicknessof the platinum surface coating is 5.67 F/cm³ [surface coating of roughplatinum 1417 μF/cm² per thickness of the platinum coating of 2.5 μm.]The platinum surface coating adhesive strength was 58 mN.

Example 2 yields a platinum surface coating having a rough surface asshown in FIG. 5. The platinum coating contains particles with veryregular particle shape and regular average size. The coating is thinnerthan known platinum coatings and has a rough surface which is mainly notporous with a large surface area. The coating provides a good adherencebetween the substrate and the platinum coating. The platinum coatedelectrode is biocompatible and therefore implantable and provides lesstissue reaction.

EXAMPLE 3 Electroplating Platinum Gray on a Conductive Substrate

A platinum polyimide electrode array having 16 electrodes (FIG. 14)having a diameter of 200 um platinum disc on the array was cleanedelectrochemically in 0.5 M H₂SO₄. The electrode array was placed in anelectroplating cell containing a plating solution having a concentration20 mM ammonium hexachloroplatinate, 0.025 M sodium dihydrogen phosphateand 0.425 M disodium hydrogen phosphate. The voltage of −0.65 V wasterminated after 30 minutes. The electrode was thoroughly rinsed indeionized water. The electrochemical capacitance of the electrode withthe surface coating of platinum gray was 1200 μF/cm², measured in a 10mM phosphate buffered saline solution. The charge/charge density andaverage plating current/current density were calculated by integratingthe area under the plating current vs. the time curve. The optimalvoltage drop across the electrodes for producing platinum gray was from−0.55 to −0.75 Volts vs. Ag/AgCl reference electrode.

The platinum coating showed the following properties:

-   -   platinum surface coating thickness: 11.0 μm;    -   electrochemical active surface area increase: 60 fold;    -   platinum surface coating adhesive strength: 50 mN; and    -   platinum surface coating color density: 1.0 D.

Example 3 yields a platinum surface coating having a fractal surface asshown in FIG. 1. The relation of surface area to the thickness of theplatinum surface coating is 1.09 F/cm³ [surface coating of roughplatinum 1200 μF/cm² per thickness of the platinum coating of 11.0 μm.]The coating provides a good adherence between the substrate and theplatinum coating. The platinum coated electrode is biocompatible andtherefore implantable and provides less tissue reaction.

The plating conditions and properties of the platinum coatings performedin Examples 1 to 3 are summarized in the following table 1. TABLE 1Conditions and Example 1 Example 2 Example 3 Properties Rough Pt RoughPt Fractal Pt Plating Agent 15 mM PtCl₄ 15 mM PtCl₄ 20 mM NH₄[PtCl₆]Plating Solution 0.025 M NaH₂PO₄ and 0.425 M Na₂HPO₄ Initial Capacitance20 μF/cm² 20 μF/cm² 20 μF/cm² Temperature 22° C. 22° C. 22° C. Voltagevs. Ag/AgCl −0.525 V −0.5 V −0.6 V Reference Electrode Time 10 minutes10 minutes 30 minutes Final Capacitance 1462 μF/cm² 1417 μF/cm² 1200μF/cm² Thickness 3.5 μm 2.5 μm 11.0 μm Surface Area Increase 73 fold 70fold 60 fold Adhesive Strength 55 mN 58 mN 50 mN Surface Coating/ 4.18F/cm³ 5.67 F/cm³ 1.09 F/cm³ Surface Thickness Current Density 11.1 A/cm²10.8 A/cm² 15.1 A/cm² Initial to Final to to to Linear Increase 15.2A/cm² 14.6 A/cm² 24.5 A/cm² Voltage Drop Across −0.4 V −0.4 V −0.55 Vthe Electrodes to to to −0.7 V −0.7 V −0.75 V Substrate Platinum 200 μm200 μm 200 μm Disc Diameter

While the invention has been described by means of specific embodimentsand applications thereof, it is understood that numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the spirit and scope of the invention. It is therefore tobe understood that within the scope of the claims, the invention may bepracticed otherwise than as specifically described herein.

1. A method for electroplating an electrode surface coating having a rough surface, comprising: electroplating the surface of a conductive substrate with platinum at a rate such that the metal particles form on the conductive substrate faster than necessary to form shiny platinum and slower than necessary to form platinum black.
 2. The method of claim 1 wherein said step of electroplating is accomplished at a rate of more than 0.05 μm per minute, but less than 1 μm per minute.
 3. The method of claim 1 wherein the electroplating process is controlled by electrode voltage.
 4. The method of claim 3 wherein the voltage is constant voltage.
 5. The method of claim 3 wherein the controlled voltage causes at least a partially diffusion-limited plating reaction.
 6. The method of claim 1 wherein the electroplating is accomplished at a rate of more than 0.05 μm per minute, but less than 1 μm per minute.
 7. An electrode surface coating prepared by the method of claim
 1. 8. A method of using the electrode of claim 1 for manufacturing a flexible stimulation device for a retina implant.
 9. A method of using the electrode of claim 1 for manufacturing high density interconnects and flexible hybrid assemblies for active biomedical implants.
 10. A method of using the electrode of claim 1 for manufacturing a thin-film multielectrode array for transmural cardiac recording.
 11. A method of using of a polymer layer of claim 1 for manufacturing thin-film multielectrode arrays for a cochlear implant. 